1. Field of the Invention
The invention is directed to a final control element for superimposed steering engagement in a vehicle steering system in which the steering angle of the vehicle wheels is varied independently of the steering wheel angle, which has advantages in terms of driving dynamics, driving safety, and travel comfort.
2. Description of the Prior Art
In a known final control element of the type defined at the outset (German Patent Disclosure DE 38 30 654 A1), the superposition gear is embodied as a single-stage planetary gear with a planet carrier, planet wheels, sun wheel and ring gear; the sun wheel is connected to the first input shaft and the ring gear to the output shaft, in each case in a manner fixed against relative rotation, and the planet carrier is in geared connection with the second input shaft via a worm gear. The first input shaft is drivingly connected to the steering wheel, and the output shaft is drivingly connected to the steered vehicle wheels, or steering gear members necessarily coupled to them. The worm gear, driven by an electric motor, is embodied as self-locking. Sensors are disposed on the first input shaft and on the output shaft, and their signals represent the rotary position of the respective shaft. The signals are delivered to a regulating device, which controls the electric motor that engages the second input shaft.
The final control element of the invention has the advantage that with the modification according to the invention of a two-stage planetary gear, the prerequisites for a superposition gear free of radial play are created, and the superposition gear can then be embodied by means of further provisions. This kind of play-free superposition gear is required, since components in the steering train must in principle be free of play. Embodying the two-stage planetary gear with sun wheels has the advantage that the sun wheels can be milled directly onto the input shaft and output shaft, and production costs are saved by omitting the expensive metal ring gears, while the length and diameter of the final control element can also be minimized.
In a preferred embodiment of the invention, means for suppressing tooth flank play between the at least one stepped planet and the sun wheels are provided. Preferably, these means for suppressing the tooth flank play have a radially resilient bearing of the at least one stepped planet on the planet carrier; preferably, at least three stepped planets are disposed on the planet carrier, one of which is supported radially resiliently. The two planet wheels pressed against a stepped planet have different angular positions in the stepped planets. The suppression of the tooth flank play that is typically present in all planetary gears guarantees the freedom from play in the transmission of rotational angles from the first input shaft to the output shaft when the electric motor is inactive. Because of the resilient bearing of one of the stepped planets, this stepped planet exerts a radial pressing force against the sun wheels meshing with the stepped planets, so that the tooth flanks always rest on one another by nonpositive engagement in every rotational position.
In an advantageous embodiment of the invention, the stepped planets are each seated on a respective shaft, whose ends are received in a respective bearing point embodied in the planet carrier, and the bearing points of at least one shaft are embodied resiliently. The stepped planets can preferably be received rotatably on their axes via a slide bearing, or the shafts can be slidingly supported in the bearing points and the stepped planets can be disposed in a manner fixed against relative rotation on their axes. The resilient embodiment of the bearing points, in an advantageous embodiment of the invention, can be achieved by providing that each of the two bearing points is disposed on a bending arm, one arm end of which is fixed spring-elastically to the planet carrier. The bending arm is embodied such that once the planetary gear is mounted, it generates a virtually radial tensing force oriented toward the meshing sun wheels.
In an advantageous embodiment of the invention, a worm gear embodied as self-locking, with a worm connected to the input shaft in a manner fixed against relative rotation and a worm wheel connected to the planet carrier in a manner fixed against relative rotation is disposed between the second input shaft and the planet carrier. Coupled to the worm wheel is a coaxial, smaller worm wheel via an annular spring, in such a way that the tooth flanks of the two worm wheels rest on both sides of the tooth flanks of the worm. By means of these structural provisions, an equalization of play between the worm gear and the planetary gear is attained, thus making a further contribution to freedom from play of the superposition gear.
In an advantageous embodiment of the invention, the worm of the worm gear is produced of plastic or aluminum. The plastic worm is either injection-molded directly onto the second input shaft or onto a metal sleeve that is pressed onto the second input shaft. Producing the worm of plastic has the advantage, besides economical manufacture, that the worm has a high noise damping capacity, which is important, since the worm rotates at high speed. Moreover, when low-density plastic is used, the centrifugal mass of the worm is not excessive, despite its relatively large diameter.
In an advantageous embodiment of the invention, the worm wheel and the planet carrier form a structural unit and are produced as an injection-molded part from one material, such as aluminum or plastic. Plastic has the advantage that the worm wheel is produced by the injection-molding process and requires no subsequent work. A plastic worm wheel is very inexpensive and furthermore has noise-damping properties. In a modification of this version, the planet carrier is made from metal, such as aluminum, and the worm wheel is made from plastic. The worm wheel is injection-molded onto the prefabricated planet carrier. These structural provisions provide increased strength of the planet carrier, greater elasticity, and less thermal expansion compared to a planet carrier made from plastic.
Overall, the refinements and improvements add up to a technically optimized, marketable final control element for superimposed steering engagement.